A common problem in the field of co-injection molding resides in the need to keep the leading edge of the core (interior) layer as uniform with respect to thickness as trailing portions of the core (interior) layer entering the mold cavity. A tapered leading edge will produce a molded part that is not uniform in its properties near the position of farthest penetration of the interior layer.
Usually, the leading edge of the core (interior) layer becomes tapered as it flows through a cylindrical central channel of prior art co-injection nozzles disposed downstream of the combining area of the nozzle and as it flows through the cylindrical gate portion of the mold cavity. Typical of such nozzles are those described, for example, in U.S. Pat. Nos. 4,895,504 and 4,892,699.
The amount of taper depends on the velocity profile of the combined flow which causes a gradient of velocity between the radially innermost portion and the radially outermost portion of the leading edge. The amount of taper also depends on the total axial distance of cylindrical flow between the area of combination and the cavity-end of the cylindrical gate.
To minimize the leading edge taper, such prior art nozzles have been constructed with a short axial flow distance between the area of combination and the cavity end of the cylindrical gate. Typically, this axial flow distance is between about 5 mm and 25 mm, and the resulting leading edge taper length is greater than about 1.8 mm for the shorter axial flow distance and 9 mm for the longer axial flow distance. This short axial flow distance requires that the combining means be part of the nozzle.
Another problem with the current art is that the outermost diameter of the co-injection nozzle close to the gate is larger than nozzle diameters used in single-material injection molding. This larger size requires a large clearance bore in the mold which makes it difficult to provide adequate cooling of the mold cavity near the gate. Some designs of current art use a combining means that has a conically or frustoconically-shaped portion to minimize the outermost diameter near the gate; even so, this diameter of the nozzle near the gate may be twice the size of a single material nozzle.